1. Field of the Invention
The present invention relates to a receiver for radio communication, such as a selective call receiver (a pager), a portable radio telephone or a cordless telephone, and in particular to a technique for restricting the effect of cross-modulation on a reception system.
2. Description of the Prior Art
Conventionally, in a common service area a plurality of carriers provide their services for mobile radio communication devices, such as selective call receivers (pagers), portable radio telephones or cordless telephones. In the service area, base stations for individual carriers are provided and facilitate communication.
In this situation, when a mobile radio device, for which a specific carrier (e.g., company A) provides a communication service, is moved near a base station of another carrier (e.g., company B), a plurality of signals transmitted by the base station of company B act as jamming signals for the mobile radio device. These jamming waves have a reception signal strength that is greater than the desired reception level for the communication service provided by company A.
In this case, even when a jamming signal transmitted by a base station of company B has a different frequency from that of a desired wave (company A), because of the nonlinear property of the receiver, the desired wave may be modulated by the jamming signal. Furthermore, when two or more strong jamming signals are present at the same time, a modulation product produced by the jamming signals is generated due to the non-linear property of the receiver, and affects the frequency of a desired wave that is received. As a result, even when the reception signal strength of the desired wave is sufficiently high, the desired wave can not be accurately received. Generally, these phenomena are known as cross-modulation and intermodulation, and are important elements that affect the performance of a radio receiver.
A conventional technique for reducing obstruction due to such cross-modulation and inter-modulation (hereinafter referred to simply as cross-modulation) will now be described while referring to FIGS. 1 and 2.
FIG. 1 is a block diagram illustrating the arrangement as a reception system of a radio receiver employing a double superheterodyne reception system that is disclosed as prior art in Japanese Unexamined Patent Publication No. Hei 5-335857.
In the conventional radio receiver, a high frequency signal is received at an antenna 51, transmitted through a variable attenuator 52, and amplified by a high frequency amplifier 53. The amplified signal is passed through a high frequency band-pass filter 54 to a first frequency mixer 55. The frequency mixer 55 mixes the received signal with a first local oscillation signal LO1 that is output by a first local oscillator 56 and obtains a first intermediate frequency signal. The first intermediate frequency signal is passed through a first intermediate frequency filter 57 to a second frequency mixer 58. The second frequency mixer 58 mixes the first intermediate signal with a local oscillation signal LO2 that is output by a second local oscillator 59 and obtains a second intermediate frequency signal. The second frequency signal is passed through a second intermediate frequency filter 60 to a wave detector 61, and is output as a demodulation signal to a demodulation section provided at the next stage.
When a strong jamming signal is received from a base station of another carrier, jamming signal induces cross-modulation on a desired wave in the high frequency amplifier 53 and the first frequency mixer 55. To cope with this phenomenon, in the receiver in FIG. 1, the variable attenuator 52 reduces the level of a high frequency signal that is input to the high frequency amplifier 53 to limit the cross-modulation that is induced by the jamming signal.
In this case, to control the degree of attenuation provided by the variable attenuator 52, the second intermediate frequency signal is amplified by an amplifier 62, the resultant signal is rectified by a rectifier 63, and the obtained direct current voltage is supplied as a gain control signal to the variable attenuator 52. That is, when a jamming signal having a high reception signal strength is received, the signal strength of the intermediate frequency signal is increased, as is the direct current voltage of the rectifier 63. Since the direct current voltage is employed as a gain control signal, the degree of attenuation provided by the variable attenuator 52 is increased.
Another radio receiver is disclosed in Japanese Unexamined Patent Publication No. Hei 5-335857. This radio receiver employs a direct conversion reception system having a simplified reception circuitry that is compactly constructed to enhance its portability, and that limits the cross-modulation that is induced by a jamming signal. In FIG. 2 is shown the configuration of this radio receiver that employs the direct conversion reception system.
In FIG. 2, the radio receiver comprises: a high frequency amplifier 72; a high frequency band-pass filter 74; a first frequency mixer 75; a local signal oscillator 76; a low-pass filter 77; a variable gain attenuator 73, for varying the strength of a received signal and outputting it along a signal path for a received signal; and a controller 79, for determining whether or not a predetermined frame sync signal is detected in base band signal data (DT) that is demodulated by a wave detector 78. If the controller 79 determines that a frame sync signal is not detected in the base band signal, a gain controller 80 generates a predetermined gain control voltage to increase the degree of attenuation provided by the variable gain attenuator 73. That is, according to the radio receiver, attention is drawn to the fact that, if cross-modulation is induced by a jamming signal, a predetermined signal is not detected in a received baseband signal of a desired wave. When it is ascertained that a predetermined sync signal is not detected in a demodulated baseband signal, the gain controller 80 reduces the gain of the variable gain attenuator 73 until it is equal to or lower than a predetermined value.
In addition, when the reception signal strength of the desired wave is reduced until it is equal to or lower than a predetermined threshold value, the gain controller 80 reduces the degree of attenuation provided by the variable gain attenuator 73 and recovers the gain. As a result, a problem can be prevented that occurs when, although normally a desired wave can be received satisfactorily, the desired wave can not be received at an adequate reception level because of the degree of attenuation provided by the variable gain attenuator 73.
However, for conventional radio receivers, the following problem has arisen concerning the control of the degree of attenuation provided by the variable gain attenuator in accordance with whether a predetermined signal is detected in a demodulated baseband signal.
Specifically, according to TIA.IS-95, which is the North American Standard for a radio communication system that employs a code division multiple access (CDMA) method using a direct sequence spread spectrum (DS-SS), a signal (a pilot channel signal) related to frame synchronization is transmitted with higher strength than another channel signal (an information signal such as a traffic signal that a user actually uses) by a base station. Thereafter, when a jamming signal having a strong signal is received, although a channel signal (an information signal such as a traffic signal) desired for reception is affected by cross-modulation, the conventional technique receives and detects a pilot channel signal having stronger strength, so that gain control is not satisfactorily exercised, and as a result, cross-modulation due to a jamming signal can not be adequately limited.
Furthermore, when cross-modulation also occurs after frame synchronization is acquired, adequate gain control can not be exercised and a satisfactory reception quality can not be maintained.
Further, the above described conventional radio receiver can not exercise adequate control when a frame sync signal can not be detected for a reason other than the occurrence of cross-modulation.